The demand for water in irrigation systems in this country has been relentless. Increasing demand for water has forced us to replace grass and vegetation with concrete, brick and rock. Wet land farming is being substituted for dry land farming, and conservation of all kinds has been encouraged to help replenish water supplies and underground springs. Technology which is capable of conserving water is constantly being sought, but there has been very little improvement, even with the advent of sophisticated new electronics.
Soil moisture detection has been used for monitoring and evaluating construction sites, landscape sites, mining operations, forest areas, flood control areas, and farming districts. The ability to determine the moisture content of granular, or packed powder media is also important in the manufacturing of cement, plaster, gravel or brick. Moisture content measurements are additionally important to sampling grain water content, field water content, and storage water content. Direct probe measurements of produce, such as growing potatoes, can provide valuable information related to moisture content, temperature, and ionic conductance activity which relate to chemical changes during the growth cycle.
Moisture sensors are often used in conjunction with sprinkler controls to regulate the flow of water in proportion to the watering needs of grass and plants. For low dielectric media, such as soil, it has been recognized that the moisture content affects the overall dielectric content of the media to a detectable degree. It is widely known that water has a relatively high dielectric constant of about 80, while dry soil typically has a much lower dielectric constant of approximately 5 or 6. The water content of soil is therefore one of the major contributors to the overall dielectric constant of the soil.
There have been many moisture sensors which have been developed for reacting to the dielectric properties of a soil sample. One popular technology employs capacitance test probes such as those disclosed in U.S. Pat. Nos. 5,445,178, 4,929,885 and 4,545,396, which are hereby incorporated by reference. No capacitive devices were found that are not strongly susceptible to effects of conductance. These devices use conductive surfaces of a probe which are imbedded in the soil. The soil is used as a dielectric, which forms a capacitor with the conductive surfaces. The capacitance provided by one of these emersed probe surfaces is used as part of an LC oscillator circuit having an oscillation frequency which varies when the dielectric property of the soil sample changes due to moisture conditions. While such devices have had limited success in measuring water content, some have had difficulty in accurately deciphering moisture contents in soils having varying degrees of salinity i.e., Salinity affects conductivity.
Early water sensing devices were not very accurate since they rarely measured the soil's capacitance without significant errors caused by temperature dependencies, or high conductivity. Saline-tainted soil and other, more unique soil types, have a definite significant effect on the measurement. The simplified electrical analogy of the medium is that of impedance, a parallel resistor and capacitor combination. Moisture sensing circuitry is therefor based on the measurement of the conductance of the resistive component and/or the reactance of the capacitive. The varying conductivity properties of the medium tends to adversely interfere with the ability of LC and RC oscillator, or frequency oriented measurement systems to accurately indicate the moisture content of the medium. Both components, created by the impedance of the medium tend to affect the oscillation frequency of some circuits, rendering it almost impossible to separate the effects of the medium's conductivity properties from those related to the medium's dielectric properties. Errors due to conductivity are further complicated by its inherent temperature dependence.
If the medium to be monitored is agricultural soil, it may contain various non-soil components, such as fertilizer, buffers, salts and the like. The complex electrical analogy of a soil/water medium results in an inaccurate indication of the moisture concentration if the circuitry is designed to produce a water content measurement based on a corresponding change in the dielectric of the medium as a function of a change in water content in the medium. Very simply, the ability of the circuitry to measure water content by the change in dielectric method, must provide evidence that the circuit is designed to isolate, remove, or be insensitive to conductivity.
Accordingly, a significant need exists for an inexpensive, water monitoring system which is capable of being sensitive to soil water content, electrical conductance and temperature even if the soil contains materials which greatly affect dielectric properties.